Frequency variation compensation circuit



Patented Feb. 9, 1943 UNITED STATE S PATE NT OFFICE FREQUENCY VARIATION COMPENSATION CIRCUIT Norman Lea, Brentwood, England, assignor to America, a corporation of Radio Corporation of Delaware Application April 27, 1942, Serial No. 440,678.

In Great Britain August a, 1940 4 Claims. The present invention relates to circuit arrangements for compensating for varying temperature coefllcient of frequency in electrical circuits, and particularly in electrical circuits which include tapped inductance elements.

. It is found that when an inductance coil is ar-* ranged for tapping'at various points, to the end that the amount of inductance which shall be included in a resonant circuit may be varied as may be required, the temperature coefficient change in frequency is not independent of the point at i which the tapping is eflected. The reasons for this are complex, but for practical purposes it may be Fig. 1a graphically illustrates the need for the invention; 2 shows the circuit of Fig. 1 embodying theinvention: Fig. 2a illustrates the functioning of the invention. I

It has been found that the curve representing the temperature coemcient or change in frequency of an inductance-capacity circuit may take the Iorm shown by the full line in Figure 1a where condenser C of Fig. 1 is assumed to have zero temperature coeflicient of change in capacity. If thetemperature coefficient of change in capacity of condenser C is altered, in the mansaid that the overall temperature coeflicient of, 1

change in frequency may be represented as a constant term and a term which is a function of the position at which the coil is tapped.

According to the present invention, 'a frequency responsive circuit including a tapped inductance includes at least two devices for compensating for variation in frequency with variation in temperature, one of said devices being 2.

adapted to compensate for the constant component of variation of frequency andat leastone other of said devices being adapted tbcom'pensate for the component of variation of frequency which depends upon the tapping point in qiifie.

-1 in use is reduced, the bulk and capacity of the overhanging part (in Figure 1 the part bee tween points 6 and 9 is the overhanging" part) is increased as is also the frequency. The increase in frequency increases the voltage generated in the mutual inductance to the overhang- The first said-device may comprise a temper- A ature compensating condenser, or a temperature compensating inductance, iifseries with or in ,parallel with the tuning condenser of the frequency responsive circuit, and -the second said device may comprise a temperature compensating condenser, or a temperature compensating inductance, connected across the lower turns of the inductance, The condensers, or inductances, comprising the said devices may at a particular temperature be of constant capacity or induc tance and adjustable temperature coeflicient of change in capacity or inductance, or they may be of constant temperature coeflicient of change in capacity or inductance and adjustable capacity or inductance, or a combination of these qualities may characterize them. There may be more than one of the second said devices, all belug-connected on one side to one tapping point, which may be one end of the inductance of the frequency responsive circuit; and each being connected on the other side to a different tapping point on the said inductance, or all being connected on both sides to different tapping points.

The invention will bedescribed further in connecticn with the drawing accompanying the Y specification wherein Fig. 1 shows a resonant circuit to which the invention is to be applied;

ing part whereby theincreased capacity of the overhanging part is charged to a higher potential. This combination of circumstances tends to introduce a square term into the efiect which the overhanging part has on the temperature coeflicient of change in frequency of the system so that the dotted curve of Fig. 1a approximates to a parabola. It is well known that if a reactance is connected across part of a resonant circuit its effect on the system depends on the square of the voltage transformation ratio, hence variations with temperature in an auxiliary condenser connected across the lower part of the inductance, e. g. between tappings ll and I can introduce a correction which moves the curve of the system from the dotted line to one which is approximately coincident with the horizontal axis so that the system has zero temperaturecoeflicient for all tappings.

In a practical embodiment of, the invention, illustrated in Figure 2 L is a tapped inductance and C 'is the main condenser. Condenser Cl is a condenser the temperature coefllcient of change in capacity of which is adjusted to give a desired datum correction to the temperature coeflicient of frequency of the system. the datum correction correspondingto the constant companent in the temperature coefllcient of frequency to the system. Condenser C2 is a condenser the temperature coeificient of change in capacity of which is adjusted to correct for the curvature of the curve representing the temperature coeflicient of change in frequency of the system. In the curves of Figure 2a, curve I is the curve of the temperature coeflicient of change in frequency for the system LC alone, and curve 2 is that when Cl is correctly adjusted. When condenser C2 is also correctly adjusted, curve 2 is straightened to become curve 3 which is coincident with the horizontal axis. Curve 4 indicates the experimentally observed reversal of curvature when the effect of C2 is too great.

As has beenindicated the adjustment of the two compensations may be achieved by making Cl and C2 of fixed capacities and adjustable temperature coeflicients of change in capacity, or of fixed temperature coefflcient of change in capacity and adjustable capacities, or a combination of these two plans may be used. In any case it is convenient to design CI and C2 for high temperature coefiicients but small capacities in order that the mean frequency of the system may not be seriously disturbed by adjustments of thermal compensation.

Instead of connecting a temperature compen sated condenser CI in parallel with condenser C, a suitably temperature-compensated condenser in series with C may be used to effect the constant component of compensation, or a suitably temperature-compensated inductance may, for this purpose, be connected in series or parallel with condenser C.

The curvature control associated with part of the coil may also take the form of an inductance or of mutual inductance.

In coil windings which exhibit more pronounced curvature in their temperature coilicient law, more than one curvature compensating reactance may be connected to the coil at suitable points.

If it is not desired to give a datum correction to the temperature coefiicient of frequency of the system, condenser Cl or its equivalent would be omitted.

I claim:

1. In a tuned circuit comprising an inductance coil having a plurality of spaced tapping points,

a tuning condenser connected between one end of the inductance and a selected one of the tapping points, at least two reactance devices for compensating for variation in frequency with temperance variation, one reactance device being in shunt with the tuning condenser, a second reactance device being connected between said one end of the inductance and a predetermined tapping point of the inductance other than said one tapping point.

2. In a frequency responsive circuit comprising a coil having a plurality of spaced tapping points and a resonating condenser connected in shunt between one end of the coil and one of said tapping points, at least two reactance devices for compensating for variation in frequency caused by temperature changes, one of the reactance devices. being electrically'connected to said resonating condenser to compensate for a constant component of variation in frequency, said second reactance device being connected between a pair of spaced points of said coil to compensate for a component of frequency variation dependent upon the inductive magnitude of the portion of said coil in shunt with said condenser.

3. In a frequency responsive circuit comprising a coil having a plurality of spaced tapping points and a resonating condenser connected in shunt between one end of the coil and one of said tapping points, at least two reactance devices for compensating for variation in frequency caused by temperature changes, one of the reactance devices being electrically connected to said resonating condenser to compensate for a constant component of variation in frequency, said second reactance device being connected between a pair of spaced points of said coil to compensate for a component of frequency variation dependent upon the inductive magnitude of the portion of said coil in shunt with said condenser, and each of said reactance devices being a condenser of small capacity and of high temperature coeflicient.

4. In a tuned circuit comprising an inductance coil having a plurality of spaced tapping points, a tuning condenser connected between one end of the inductance and a selected one of the tapping points, at least two reactance devices for compensating for variation in frequency with temperature variation, one reactance device being in shunt with the tuning condenser, a second reactance device being connected between said one end of the inductance and a predetermined tapping point of the inductance other than said one tapping point, and each of said reactance dewces being a condenser of relatively small capacity and of relatively high temperature coefficient.

NORMAN LEA. 

